Prior to forging, the powder in the above described method has a relative density of not more than about 70%, while the capsule material is completely homogenous. When one starts forging on the capsule, the powder initially is compacted but nothing else happens with the capsule than its being deformed, often to an irregular shape. The capsule in other words becomes too large for its contents. During the initial phase of the forging process, the powder, therefore, will be compacted while the capsule only will be "wrinkled". During this initial phase the relative density of the powder will increase from about 70% to a about 97 to 98%, as is the case when, by way of example, a high alloyed tool steel is concerned. The problems with the "wrinkling" to some extent can be overcome by using a forging machine of the type having tools movable in four radial directions as according to U.S. Pat. No. 3 165 012, or by forging in a closed tool as according to U.S. Pat. No. 4 038 738. If, on the other hand, a round capsule shall be forged to a shape having a square section, the complex of problems will be accentuated. This also concerns the next phase of the process when the powder body is almost dense. During this phase the powder body begins to elongate, very much in the same mode as a homogenous material would do. The contents in the capsule then begins to fill the capsule again and there is an aim during the whole of this phase to keep contact between the capsule wall and the contents in the capsule in order to avoid cooling of the capsule which would cause inhomogenities in the material.
In the described process the end plates, which are welded to the ends of the cylindrical walls, are deformed during the forging operation, such that the end portions are strangely wrinkled, which is a great problem. If, by way of example, a flat end plate is welded to the inside of the cylindrical wall, as is shown in FIG. 1, the end wall will bulge as indicated by the arrows. When the forging is finished the weld will be positioned on the cylindrical wall. This implies that the weld has been bent 90.degree. from the rear side of the weld in the weld zones which are very sensitive to cracking. At the same time the weld is subjected to great tensile stresses. As a result the weld joints will easily crack so that air can penetrate the interior of the capsule before the material has become completely dense. The powder will thus be oxidized, which will cause refusal.
In order to avoid bendings and heavy tensile stresses in the weld joints one may instead consider to make the weld joint in the region of the cylindrical wall from the very beginning. The end plate for example may be designed as the end wall of a pressure vessel, FIG. 2. In this case, however, it will be difficult to completely fill the capsule with powder, which creates a secondary problem. It is, per se, easy to fill a capsule on a small scale, but it is more difficult to do it in practice. The angle of repose of the powder is namely such that it is difficult to fill the powder all the way into the corners, even if a hole is made in the end wall which later is plugged up.
In experiments with end plates according to FIGS. 1 and 2 the end plates bulged already in the first forging steps. As already mentioned the capsule initially will be "too big". If at the same time the end walls will bulge, the capsule will become much bigger than the not completely compacted powder body. This implies that the capsule will get poor contact with the powder at the ends. This in its turn implies that the end portions of the capsule will be cooled. The end portions, in other words, will become cold and rigid while the cylindrical wall portion will remain hot and ductile. Between, on one hand, this cold and rigid end portions and, on the other hand, the hot and ductile cylindrical wall the weld joint is situated, which therefore also in this case runs the risk of being subjected to such heavy stresses that it may crack. In this case there is thus a risk that the capsule will break at the weld joint and also a risk for inhomogenities in the material because of lost contact between the capsule and the powder body.